Spectrograph with multiplexing of different wavelength regions onto a single opto-electric detector array

ABSTRACT

An optical spectrograph utilizes a plurality of holographic transmission optical gratings operative to receive an incoming source of light to be analyzed and diffract the light such that different spectral components impinge upon spatially separated regions of an opto-electronic detector. Various grating configurations are disclosed, including a physical stack of gratings conducive to extreme compactness, as well as a spaced-apart configuration used to preclude spectral cross talk in certain configurations. Diverging light emerging from a fiber-optic bundle is collimated by a first lens assembly prior to passing through the gratings, and a second lens assembly is used to focus the diffracted light onto the detectors, preferably in the form of a two-dimensional CCD array.

This application is a continuation of application Ser. No. 08/050,862,filed Apr. 21, 1993, now U.S. Pat. No. 5,442,439.

FIELD OF THE INVENTION

This invention relates generally to methods and apparatus applicable tospectroscopy and, in particular, to a spectrograph configuration whereindifferent wavelength regions are multiplexed such that they impinge ontoa single detector array in spatially separate areas.

BACKGROUND OF THE INVENTION

A spectrograph is a device for recording the spectral composition oflight emerging from an entrance aperture, such as a slit. A simple,prior-art spectrograph configuration uses a conventional Czerny-Turnermonochromator consisting of a planar reflection grating and a pair ofconcave spherical mirrors. Radiation emerging from the entrance slit iscollimated by the first mirror, and the collimated light ischromatically dispersed by the grating to separate the color contentalong different angles. The second mirror then focuses these angularlydispersed colors into spatially separated images, and an exit slitpasses only a small color range for measurement by a single detectorchannel. Data may be gathered over an extended spectral range byrotating the grating, which causes different color regions of thespectrally dispersed image to coincide with the exit slit.

Spectrograph configurations based upon the Czerny-Turner monochromatorare slow due to the serial nature of the spectral data acquisition.Speed limitations are further pronounced in applications such as Ramanspectroscopy, where weak signal levels require long integration timesfor each spectral data point on a photon-counting detector.

Recent advances have resulted in improvements to both spectrumacquisition time and the signal-to-noise ratio in spectrographicinstruments. For example, spectral data may be acquired in parallel on atwo-dimensional detector, which replaces the exit slit, thus allowingthe grating to remain fixed while the detector array is illuminated byan extended spectral range from the monochromator. The grating may thenbe stepped to another position or replaced with a different grating toacquire data in a different spectral range.

If a simple entrance slit is being imaged onto a CCD detector, thedetector signals may be added or binned in the constant color directiondirectly on the detector device, thus enhancing sensitivity andsignal-to-noise ratio. If increased spatial resolution is also required;for instance, if the entrance slit is replaced by an array of opticalfibers from different light sources, then a modified imagingCzerny-Turner monochromator may be constructed in conjunction with a 2-Dimage sensor by replacing the spherical mirrors with toroidal mirrors tocorrect for astigmatism. Another recent development in spectrographtechnology is the use of volume holographic transmission gratings inplace of the more conventional surface relief reflection gratings ofboth the ruled and holographic type. Such volume gratings can be usedwith on-axis transmission lenses as imaging elements in place of thereflective mirrors, resulting in an efficient system and a very compactpackage as evident by U.S. Pat. No. 5,011,284, assigned to the assigneeof the present invention. Volume gratings are also capable of very highdispersion, enhancing the spectral resolution for a given focal lengthand image size.

While the use of a detector array in place of an exit slit enhancesspectrographic performance and reliability, the use of a rotatablegrating, grating turret, or any moving component remains a seriousdrawback. Thus, even with recent advances in the prior art, thereremains an unsatisfied need for a compact and efficient spectrographicinstrument wherein a two-dimensional detector array may be employed forboth high spectral resolution and a large spectral bandwidth without theneed for moving parts.

SUMMARY OF THE INVENTION

The present invention builds upon previous spectrographic concepts andsolves prior art limitations through the use of holographic transmissiongratings and a two-dimensional detector array which, in combination,extend spectral range and/or resolution. The preferred embodimentutilizes a plurality of holographic transmission optical gratings, eachhaving a different line spacing to disperse a particular portion of thespectrum, and means for directing a portion of the light to be analyzedonto each grating so as to diffract at least a portion of the incidentlight onto a spatially separate region of the detector, so thatdifferent areas of the detector receive signals representative ofdifferent portions of the spectrum encompassed in the incoming beam. Inthe preferred embodiment, the invention configures the gratings as astacked structure, each grating exhibiting a diffraction path tiltedslightly with respect to the paths of the other gratings, enablingwavelength bands representative of the incoming radiation to impingeupon the detector as adjacent or stacked striped regions.

In an alternative embodiment, the gratings are not stacked but areparallel and spaced apart so that light diffracted by one grating is notattenuated by a subsequent grating, thereby minimizing any potentialcross talk problems. This alternative configuration may also beconstructed in a more modular fashion, allowing different combinationsof "drop-in" gratings to be more expressly changed than would bepossible with a fixed sandwich-like structure.

In both embodiments the tilting of the diffraction paths may be derivedby either recording a tilt angle in the fabrication of the gratings orby physically tilting the gratings with respect to one another.

In addition to the plurality of gratings, a lens assembly is used tocollimate the incident light prior to dispersion by the holographicgratings, and a second lens assembly is used to focus the spectrally andspatially separated light onto the detector array, preferably atwo-dimensional CCD. In the alternative embodiment utilizing spacedapart gratings, the focusing lens requires an enlarged input aperturesince the various gratings are accessing different regions of tilecollimated input beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a preferred layout of wavelength regions across the surface ofa CCD detector;

FIGS. 2 and 2a are drawings of a spectrograph formed in accordance withthe present invention wherein two transmission holographic gratings arephysically stacked;

FIG. 3a is a top view of two holographic gratings useful in the presentinvention which exhibit fold angles of approximately 90°;

FIG. 3b is a side view of the gratings of FIG. 3a illustrating theirdifferent tilt angles;

FIG. 4a shows the diffraction efficiency of two ideal gratings usefulwith the present invention;

FIG. 4b illustrates curves showing the practical diffraction efficiencyof two gratings useful in the present invention;

FIGS. 5a-b are drawings of alternative embodiments of the presentinvention wherein two gratings are parallel but spaced apart such thatlight diffracted by grating 1 is not affected by a subsequent passthrough grating 2;

FIG. 6 is a drawing of the second alternative embodiment of the presentinvention utilizing a multiplexed input aperture; and

FIG. 7 is yet another alternative embodiment of the present inventionwherein adjacent gratings are planarly formed onto a single substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a portion of the surface of an imagedetector having a plurality of opto-electric detectors 12, in thepreferred case this sensor being a charge-coupled device (CCD) and withthe opto-electric elements being the pixels of the CCD. Alternatively,the detector could be a charge injection device (CID), intensified diodearray, or any two-dimensional array detector. Two regions of theincoming optical radiation to be analyzed impinge upon the surface ofthe CCD, the first wavelength region 14 spanning wavelength λ₁ to λ₂,and the second wavelength region 16 beginning with wavelength λ₂ andspanning to λ₃. The vertical slices 18 making up each wavelength regionrepresent the monochromatic images of the light emerging from theentrance aperture, in the preferred embodiment this being a stack ofoptical fibers carrying radiation that was scattered by a sample orsamples of interest located remotely from the apparatus of the presentinvention.

While FIG. 1 shows two wavelength regions stacked such that theirlongitudinal sides are adjacent, it should be noted that otherplacements of the wavelength regions onto the detector array arepossible, and that the number of such regions is limited only by theresolution desired; that is, the number of pixels dedicated to eachwavelength region, and, of course, the total number of opto-electronicdetectors available in the detector array.

FIGS. 2 and 2a show preferred configuration of a preferred configurationof the components which comprise the spectrograph of the presentinvention, those components being indicated generally at 20. Lightscattered by a remote sample is in this case carried along fiber bundle22, where it emerges at a surface 24 formed at the end of the bundle andilluminates, as indicated by diverging lines 26, a first lens assembly28, which collimates the incoming radiation as indicated bysubstantially parallel lines 30. The collimated light impinges upon aplurality of stacked holographic transmissive optical gratings indicatedgenerally at 32, with the light first striking a grating 34 associatedwith the first wavelength region 14 depicted in FIG. 1, spanningwavelengths λ₁ to λ₂.

Grating 34 diffracts the portion of incident light 30 that lies in thewavelength region λ₁ to λ₂ through an angle of approximately 90°.Incident light outside the wavelength region λ₁ to λ₂ is transmittedwith little or no diffraction by grating 34. Both diffracted andtransmitted portions of the light leave grating 34 and are immediatelyincident to a second grating 36, which is associated with the secondwavelength region 16 depicted in FIG. 1, spanning wavelengths λ₂ to λ₃.The light diffracted by grating 34 is outside the wavelength region ofgrating 36, and is therefore transmitted with little or no diffractionby grating 36. Light within the wavelength region λ₂ to λ₃ that wastransmitted by grating 34, on the other hand, is diffracted by grating36 through an angle of approximately 90°.

In this preferred configuration, each grating performs two angularredirections of the incoming radiation, the first of these redirectionsbeing depicted by a fold angle 40 which, in this case, is approximately90° in the case of both gratings. Each grating also performs aredirection represented by a tilt angle which may be thought of ascoming out of the page in the case of the first wavelength region andinto the page in the case of the second wavelength region. This criticalconcept will become increasingly understood through subsequentdiscussions in conjunction with other figures.

The incoming radiation, having been spectrally separated into wavelengthregions, passes through a second lens assembly 50, which is operative tofocus the wavelength regions into spatially separate areas on thedetector device 10. As the view provided in FIG. 2 is a top view, thespatially separated wavelength regions are stacked into the page of thedrawing, an arrangement which is evident by the side view 52 representedby A--A in the figure.

Turning now to FIG. 3, FIG. 3a is a top view depicting the path of thecollimated incoming radiation, depicted by lines 30 as it passes throughgrating 34 on the left responsible for the first wavelength section, andgrating 36 on the right responsible for the second wavelength section.The angle 40 formed by the incoming radiation and the radiation afterbeing diffracted by gratings 34 and 36, as represented with lines 60 and62, is the fold angle introduced in FIG. 2. As mentioned, in thepreferred embodiment, this angle is approximately 90°, but mayalternatively, in fact, be any angle above or below to 90°; that is tosay, the light may pass through the gratings and be redirected only interms of the tilt required to position different wavelength regions ontodifferent areas of the image sensor and the dispersion required tospread each wavelength range across the extent of the array and still bein keeping with the present invention. Moreover, although the foldangles for the two gratings shown are substantially equal, the foldangle for each grating may, in fact, be different. Additionally, whilethe gratings are shown in air, they may be placed in glass or any mediumexhibiting a desirable index of refraction in accordance with theapplication at hand. In summary, the fold angle for each grating and themedium index associated with the gratings may be chosen in the contextof the overall system design, including consideration of focal length,fiber size, CCD dimensions, desired band widths, and so on.

Another significant consideration is polarization. The 90° fold with thegrating in glass will not efficiently diffract P-polarized light, thougha CCD-based spectrograph should not need or want that much dispersion. Alesser fold angle in glass, or a fold in air as shown, providessignificant P-polarized throughput.

Turning now to FIG. 3b, a side view of gratings 34 and 36 illustratesthe tilt angles which are indicated by numerals 70 and 72 respectively.In this preferred arrangement, incoming radiation depicted by lines 30is tilted slightly upwardly in the figure at angle 70 by element 34,whereas the incoming radiation associated with the second wavelengthsection is tilted slightly downward at angle 72 by element 36. In thiscase, the tilt angles are roughly similar and symmetrical about thecentral axis of the incoming radiation; however, various tilt angles arepossible, including multiple tilt angles to either side which, in fact,would be necessary if more than two gratings are used.

FIG. 4 illustrates the diffraction efficiency of two gratings useful inthe preferred embodiment of the apparatus. In FIG. 4a, an idealdiffraction efficiency is depicted, wherein the grating associated withthe first wavelength section diffracts the entire band of wavelengthsfrom λ1 to λ2 as shown by line 80, yet transmits virtually unaffectedthe second wavelength section comprising wavelengths λ2 to λ3, asindicated with line 82 being nearly zero. In contrast, the secondgrating, shown on the right side of FIG. 4a, performs oppositely to thatof the first grating by transmitting unaffected the first wavelengthsection as indicated by the near-zero line of 84 yet diffracting allwavelengths from λ2 to λ3 as indicated with the line 86. The practicalversion is better represented by the curves of FIG. 4b, wherein thegreatest diffraction efficiency for a particular band is about (λ1+λ2)/2for the first grating and (λ2+λ3)/2 for the second grating, theseaverage values being shown by broken lines 90 and 92 respectively. Inaddition, with real-world elements there will be some overlap betweengratings as shown by the hashed regions 94 and 96.

Turning now to FIGS. 5a-b, an alternative configuration of the presentinvention is depicted generally at 100, wherein the gratings are notphysically stacked as in FIG. 2 but are spaced apart at positions 102and 104, respectively. As in the preferred embodiment, incoming light 26diverging from surface 24 of fiber 22 passes through a first lensassembly 28 as collimated light depicted by lines 30, and passes througha first grating 34 and a second grating 36. In this case, however, sincethe second grating is physically spaced apart from the first grating,the light diffracted by the first grating, as indicated generally bylines 106, does not pass through the second grating and is thus notaffected thereby. This configuration thus eliminates cross talk problemsthat may be presented by physically stacking the gratings, and may allowa more modular configuration, facilitating different combinations of"drop-in" gratings to be more easily utilized and exchanged. Moreover,as shown in the side view, the tilt angle need not be recorded in thegrating; instead, the gratings may be physically rotated to provide thetilt required to direct different wavelength sections onto differentareas of the image sensor. This concept may be extended by having eachgrating rotated relative to the other gratings by angle 110 shown in theside view. This alternative configuration does present two potentialdrawbacks, however, the first being that it is less compact than thearrangement possible with stacked gratings, the second being the needfor a second lens element 110 having twice the aperture or speed of thelens element in the stacked-grating configuration, since in this caseeach grating is accessing a different portion of the lens aperture.

FIGS. 6 and 7 show two more alternative embodiments of the presentinvention. FIG. 6 uses a multiplexed input aperture, and arranges thetwo gratings such that the spectral band associated with each uses onlyone half of the available input aperture. In FIG. 7, adjacent gratingsare formed on a single substrate in conjunction with a faster inputaperture. While an advantage of this configuration is the completeelimination of cross talk, either faster collimating and focusing lensesare both required, or the system will effectively operate at a lowernumerical aperture.

We claim:
 1. An optical spectrograph for use in analyzing a plurality ofincoming light beams, comprising:a two-dimensional opto-electricdetector array containing rows and columns of detector elements; aplurality of holographic transmission optical gratings, each recorded todiffract a predetermined range of wavelengths, said gratings beingsupported so that each light beam to be analyzed passes through each,with each grating diffracting wavelengths of each beam in its range ontoa different row of said detector array, resulting in multiple bands ofradiation incident upon said array, each band having a height in pixelsrelated to the number of incoming beams.
 2. The optical spectrograph ofclaim 1 wherein said incoming light beams are arranged in a planeparallel to one another at least where they pass through said gratings.3. The spectrograph of claim 1 wherein the recording of a particulargrating alone causes the light diffracted by that grating to fall onto aparticular set of detector elements comprising said array.
 4. Thespectrograph of claim 1 wherein the recording of a particular gratingtogether with its physical orientation cause the light diffracted bythat grating to fall onto a particular set of detector elementscomprising said array.
 5. The optical spectrograph of claim 1 whereinsaid gratings are substantially planar and stacked parallel to oneanother.
 6. The optical spectrograph of claim 1 wherein said gratingsare spaced apart such that light diffracted by each grating isre-directed so as not to pass through subsequent gratings.
 7. Theoptical spectrograph of claim 1 where said opto-electric detector arraycomprises a charge-coupled device.
 8. The optical spectrograph of claim1 wherein said incoming light beams are carried by separate opticalfibers.
 9. The optical spectrograph of claim 1 wherein said incominglight beams are carried by separate optical fibers in a flat ribbon ofsuch fibers.
 10. The optical spectrograph of claim 1, further includingmeans to collimate said incoming light beam.
 11. The opticalspectrograph of claim 1, further including means to focus the lightdiffracted by said gratings onto said detector elements.
 12. In anoptical spectrograph of the type including a holographic transmissionoptical grating operative to diffract light to be analyzed onto anopto-electric detector, the improvement comprising:means forsimultaneously receiving a plurality of light beams to be analyzed; saiddetector is in the form of a two-dimensional image sensor; and aplurality of said gratings, each having a different line spacing andeach exposed to each beam to be analyzed, and means to direct the lightof each beam diffracted by each grating onto a different surface area ofsaid sensor.
 13. The optical spectrograph of claim 12, wherein said forsimultaneously receiving a plurality of light beams to be analyzedincludes a ribbon of optical fibers, each carrying a different beam. 14.An optical spectrograph, comprising:a plurality of light sources to besimultaneously analyzed, the light from each source passing through aplurality of holographic transmission optical gratings, each gratingbeing operative to diffract the light received at a different anglerelative to the other gratings; a two-dimensional image sensor; andmeans for directing the light diffracted by each grating onto thesurface of said sensor, whereby signals representative of differentportions of the spectrum encompassed in the light from each sourceimpinge upon said sensor in different areas.
 15. The opticalspectrograph of claim 14 wherein the different portions of the spectrumencompassed in the light from each source impinge upon said sensor as aseries of separate bands having a height related to the number ofsources, the width of each band being representative of a differentspectral range.
 16. The method of simultaneously spectrally separating aplurality of light beams, comprising the steps of:passing the opticalradiation of each beam through a plurality of holographic transmissionoptical gratings, each grating diffracting a portion of the opticalradiation of each beam using a different line spacing; and focussing thediffracted light onto a planar image sensor so that the radiationdiffracted by each grating falls on a different row of the sensor. 17.The method of claim 16 wherein the gratings are planar and supportedparallel to one another.
 18. The method of claim 16 wherein saidgratings are oriented angularly with respect to one another so that theradiation from each grating falls on a different area of said sensor.